It is generally known that undesirable emissions of internal combustion engines may be significantly reduced by passing the exhaust gas thereof through catalytic converters. When conventional converters are at light-off temperatures, such as 350 degrees Celsius, exothermic reactions occur therein, resulting in efficient reduction of engine emissions. Converter temperature may be elevated to light-off temperature from latent exhaust gas heat. On cold start, a converter heated with latent exhaust gas heat may, under normal engine warm-up conditions, require 75 seconds or more to reach its light-off temperature.
Additional heat sources have been proposed for more rapid heating of the converter, so that efficient conversion of undesirable exhaust gas constituents may begin earlier in the operating cycle. For instance, electrically heated converters have been proposed, in which electrical energy is selectively applied to a device that converts that electrical energy to thermal energy, which is made available to the converter. The efficiency of such heating systems requires that substantial electrical energy be expended before any significant emission reductions may be achieved.
Alternatively, burner systems have been proposed for rapid heating of the converter. Such systems attempt to ignite an air/fuel mixture at a point slightly upstream of the converter, whereby the released combustion energy operates to rapidly increase the converter temperature. The combustion of the air/fuel mixture in such systems may be provided for and maintained by a periodic spark in proximity to the air/fuel mixture, such as from a conventional spark plug driven by a periodic energization signal from a controller.
In a first mode of operation, burner systems typically attempt to initiate combustion in the burner as rapidly as possible. A substantially rich air/fuel mixture is provided to the burner in proximity to a periodic spark. Once combustion is present, a second mode of operation is entered, in which the air/fuel mixture may be enleaned consistent with a compromise between the competing goals of rapid heating of the converter and favorable emissions from the burner-converter combination. To enter the second mode of operation as soon as combustion is taking place in the converter will contribute to these goals. Accordingly, what is needed is a system for rapidly detecting combustion in the burner and for indicating the presence of such combustion.
Furthermore, in the second mode of operation, it is desirable to monitor the combustion process, to ensure that in the event burner combustion is extinguished, the supply of fuel is stopped. Otherwise, a substantial quantity of unburnt fuel may be passed through to a catalytic converter that is not yet at light-off temperature, resulting in increased levels of undesirable emissions to the atmosphere. Accordingly, what is also needed is a system for continuously monitoring combustion activity in the burner, and for rapidly indicating an absence of combustion therein.